1. Field of the Invention
The present invention relates to an image forming apparatus for developing an electrostatic charge image and magnetic latent image in, e.g., an electrophotographic method, electrostatic printing method, and magnetic recording method, and a developing agent for use in this image forming apparatus; and, more particularly, to an image forming apparatus using a heat fixing system such as heated roller fixing, and a developing agent for use in this image forming apparatus.
2. Description of the Related Art
An electrophotographic method generally uses a photoconductive material to form an electrostatic latent image on a photoreseptor by using various means. This electrostatic latent image is developed using a developing agent, and the developing agent image is transferred onto a transfer medium such as a paper sheet where necessary. A copied image is obtained by fixing the transferred image by, e.g., heat, pressure, heat and pressure, or solvent vapor. The developing agent not transferred but remaining on the photoreseptor after the formation of the copied image is removed by various methods, and the above process is repeated.
Recently, such an image forming apparatus for forming a copied image is being strictly required to exhibit decreased size and weight, reduced power consumption, and high reliability. As a consequence, the performance required of the developing agent is also advancing.
In a heat fixing system which performs fixing via a heating roller or film in the fixing process, the surface of the heating roller or fixing film is formed by using a material which imparts releasability to a developing agent, and fixing is performed by moving a transfer medium such that the surface of the material is in contact with the surface of the roller or film. In this method, the thermal efficiency when the developing agent image is fused on the transfer medium is very high because the surface of the heating roller or fixing film is in contact with the surface of the transfer medium, so the image is rapidly fixed. In this heat fixing method, however, the surface of the heating roller for fixing film is in contact with the developing agent image in a molten state. Therefore, a portion of the developing agent image sometimes adheres and transfers to the surface of the heating roller or fixing film, and then transfers to the subsequent transfer medium to cause a so-called offset phenomenon, thereby contaminating a fixing sheet. Accordingly, one problem to be solved of this heat fixing method is to prevent adhesion of the developing agent to the surface of the heating roller or fixing film.
In a heated roller fixing system which performs heat fixing by using a heating roller, various attempts for reducing the fixing energy have been made as recent energy-saving measures. As a method of reducing the fixing energy, various attempts have been made to reduce the quantity of heat (heat capacity) of the heating roller itself, thereby reducing the time before heat accumulation and increasing the heat transfer efficiency. More specifically, decreasing the thickness of the core metal of the heating roller eliminates the need for a large amount of heat and reduces the heat capacity. However, when a heating roller having a thin core metal is used, the temperature of the roller surface significantly decreases when a paper sheet passes by because the heat capacity of the heating roller is low. Accordingly, it is necessary to maintain the roller surface at a predetermined temperature by continuously heating the heating roller by a heating member, thereby replenishing the heat deprived of by the passing paper.
A developing agent for use in a fixing apparatus including the heating roller as described above is required to have good adhesion, i.e., good fixing properties with respect to a transfer medium over a broad temperature range.
Conventionally, to prevent adhesion of toner to the surface of the heating roller, the roller surface is formed by using a material, e.g., silicone rubber or fluorine-based resin, having high releasability to the toner. In addition, to prevent offset and fatigue of the roller surface, the roller surface is coated with a liquid having high releasability, such as silicone oil.
This silicone oil coating is very effective to prevent offset of the developing agent. However, an apparatus for supplying the silicone oil is additionally required, and the fixing apparatus becomes complicated. Also, if the silicone oil evaporates by heat, the interior of the machine is contaminated.
On the other hand, to supply a highly releasable material from the developing agent without supplying any silicone oil, a method of adding a releasing agent such as low-molecular-weight polyethylene or low-molecular-weight polypropylene to the developing agent is proposed. However, if a large amount of releasing agent is added to achieve a satisfactory effect, filming to the photoreseptor, contamination of the developing roller surface, or the like occurs, and this degrades the image.
To avoid this, a small amount of releasing agent by which no image degradation occurs is added to toner, and at the same time a slight amount of releasing oil is supplied or an apparatus which removes offset developing agent by using a winding type member, e.g., a web is used.
A developing agent to which wax is added as a releasing agent is also known. This wax is used to increase the offset resistance when toner is at lower temperatures or high temperatures, and improve the fixing properties at low temperatures. Unfortunately, the wax worsens the blocking resistance against the developing agent, or worsens the developing properties when heated by a temperature rise of a copying machine or the like. Also, when left to stand for long periods, the wax blooms to worsen the developing properties.
In addition, Jpn. Pat. Appln. KOKAI Publication No. 8-106173 discloses a developing agent which uses a combination of wax and a predetermined binder resin, e.g., a resin having an acid value, in order to improve the low-temperature fixing properties and the offset resistance.
As described above, conventional developing agents are superior in high-temperature offset and developing properties but inferior in low-temperature fixing properties, or superior in low-temperature offset and low-temperature fixing properties but inferior in blocking resistance. Also, the developing properties worsen with a temperature rise in the machine, and low-temperature and high-temperature offset resistances cannot be achieved at the same time.